This invention relates to a generator which generates an output signal indicative of the rotational speed of a rotating body, such as the shaft of an electric motor, in order to control the rotational speed of the rotating body with a feedback circuit.
Various types of generators or rotational speed detectors or sensors have been known, and in electric motors for driving the capstan of a tape recorder, a rotational speed indication signal generator of the type arranged to produce an output signal by electromagnetic induction is known. Such a conventional generator comprises a rotor and a stator which constitute a magnetic circuit having a permanent magnet, and a coil which generates an output signal as the rotor rotates by detecting the change in magnetic flux passing therethrough. This conventional generator, however, is apt to be influenced by external leakage flux which comes from the motor, in which the generator is built, a power transformer, and an electromagnetic plunger or the like which are disposed in the vicinity of the motor. Namely, the generating coil of the generator is apt to pick up leakage flux from (a) magnetic circuit(s) other than that of the generator, so that undesirable output signal results.
Therefore, such a conventional rotational speed indication signal generator is not suitable for a motor whose rotational speed is relatively low because signal voltage induced during low speed rotation is relative low, resulting in low S/N of the output signal.
According to a conventional technique, another coil is provided for removing the undesirable electrical output caused by external leakage flux. Namely, a second coil is axially spaced from the generating coil of the generator so that a second coil does not interlink with the flux passing through the magnetic circuit of the generator, and the other coil is electrically connected to the generating coil so that the undesirable electrical signal induced across the generating coil is cancelled or offset. Although such a generator having a compensating coil is practically used, its structure is complex, and the manufacturing cost is high. Furthermore, the undesirable influence due to leakage flux cannot be perfectly removed with such a compensating coil.